[Notifier] Move abstract test cases to namespace

This enables BC promise for the abstract test classes, like for other ones.

As we ignore current `/Tests` directory in `.gitattributes`, builds will fail if `symfony/notifier` is installed as dist.

Author: OskarStark

Tests/LinkedInTransportFactoryTest.php

@@ -3,7 +3,7 @@
 namespace Symfony\Component\Notifier\Bridge\LinkedIn\Tests;
 
 use Symfony\Component\Notifier\Bridge\LinkedIn\LinkedInTransportFactory;
-use Symfony\Component\Notifier\Tests\TransportFactoryTestCase;
+use Symfony\Component\Notifier\Test\TransportFactoryTestCase;
 use Symfony\Component\Notifier\Transport\TransportFactoryInterface;
 
 final class LinkedInTransportFactoryTest extends TransportFactoryTestCase

Tests/LinkedInTransportTest.php

@@ -11,7 +11,7 @@
 use Symfony\Component\Notifier\Message\MessageOptionsInterface;
 use Symfony\Component\Notifier\Message\SmsMessage;
 use Symfony\Component\Notifier\Notification\Notification;
-use Symfony\Component\Notifier\Tests\TransportTestCase;
+use Symfony\Component\Notifier\Test\TransportTestCase;
 use Symfony\Component\Notifier\Transport\TransportInterface;
 use Symfony\Contracts\HttpClient\HttpClientInterface;
 use Symfony\Contracts\HttpClient\ResponseInterface;